The (3He, α) reactions on even titanium isotopes

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  • 2 [ Nuclear Phystcs A100 (1967) 401--415, (~) North-HollandPubhshmg Co Amsterdam B 2 G I

    Not to be reproduced by photoprmt or microf i lm wLthout v, n t ten permlss~on f rom the pubhsher

    THE (3He, ~) REACTIONS ON EVEN TITANIUM ISOTOPES

    J L'ECUYER and C St-PIERRE Physws Department, Umt erstte Laral, Qtwbec, Canada *

    Received 9 May 1967

    Abstract The angular distnbutmns of the alphas from the reactmns a6Yl(3He, ~)4~T1, 48Tff3He, aWTL and 5Tl(6He, ~)49T1 have been measured at E3H~ -- 10 Me~ A number of In :- 3 transmons corresponding to the pick-up of neutrons from the lf~ shell, has been observed We have also found In ~ 0 and 2 transmons, indicating that the pick-up can also originate from the 2s~ and ld~ shell Finally some In ~ 1 transltmns have been observed, indicating p-wave admixture lO_ the ground state of the e~en t~tamum isotopes SpectroscopLc factors have been deduced for the various transmons by means of a DWBA analys~s The experimental results have been compared with the theoretical pred~cUons of the shell model and of the rotatmnal model with Corlohs couphng

    E NUCLEAR REACTIONS 46,~s,5Tl(3He, ~), E ~ 10 MeV, measured ~(0) 4.,~-49T1 deduced levels, In, spectroscopic factors Enrtched targets

    1. Introduction

    Nuclet in the f: shell have recently been the object of considerable theoretical work

    However, those nuclei are still far from being well known and experimental work ts

    needed to test the different models which are presently proposed Accordingly we decided to study the even t itanium isotopes using the (3He, ~) reaction The results

    of this investigation were analysed using the DWBA Spectroscopic factors were then obtained and compared with the various theoretical predictions

    Previous investigations of the t l tamum tsotopes revealed some rather puzzhng fea-

    tures Since an (3He, ~) reaction amounts to the pick-up of a neutron from the target

    nucleus, one expects that the neutrons will come mainly from the f, shell which is m

    the process of being filled It should also be possible to observe neutrons from the

    s-d shell which is the least bound closed shell One then expects to observe mainly

    1 = 3, 2 and 0 angular momentum transfer and possibly some rather weak l = l

    tt ansmon coming from 2p admixture m the ground state of the even htamum isotopes

    This was observed prewously m the (p, d) work of Kashy and Conlon 1) and of Sherr

    et a/ 2) and also m the (d, t) mveshgahon ofYntema 3) At the same time tt was found that some of the hole states excited by the pick-up of a neut lon m the s-d shell were also clearly seen m the (d, p) work of Rapaport et al 4) This observation would then

    indicate that the ground state of the even t l tamum isotopes Is more comphcated than

    Research supported by the Atomic Energ~ Control Board of Canada

    401

  • 402 J L ECUYER AND C ST-PIERRE

    was first thought because in addition to having 2p admixture, one should also add terms coming from holes m the s-d shell

    In the present lnvestlgaUon we took advantage of the high positive Q of those (JHe, ~) reacUon to look for as many levels as poss%le and we Uied to deduce the occupation number of the various sub-shells In addition m some cases we should then be able to compare our result with the pievmus (p, d) work Since spectroscopic lectors should be identical for (p, d) and (3He, ~) reactions, a comparison between otu result and the one of refs i, 2) prowdes a good test of the rellabflmty of the occupa- tion number which we deduce

    2. Experimental procedure

    The experiment ~as done using the 3He + beam of the Laval 5 5 MV Van de Glaaff The energy was kept fixed at 10 MeV by an analysing magnet A quadrupole lens and a series of slits at the enU ance of the scattering chamber were used to focus and define the beam The beam was stopped in a Faraday cup and the current was continuously monitored Because of the large cross section for elastic scattering at 10 MeV it was necessary to limit the cmrent on the target to values which range from 5 to 500 nA as the angle is increased from 15 ~ to higher values

    The alphas were observed with solid state detectors Most of the small angle meas- uIes x~ere carried out using two AE counters, one of 100/an and the other of 120 Itm m a telescopm ai rangement The first detectm was used to stop the scattered 3He and the second was thick enough to stop all the alphas A fast coincidence module gave the signal required to analyse the pulses coming out of the detectors An antlplle-up-

    TABLE l

    Isotopzc composi t ion of the targets and (aHe, ~) Q-values

    Target ;',, a'T1 "o arT1 o 48Tx ,,o 40Tl o~, oOTl Q(Me\ ) (ref 5)

    48T1 81 2 2 1 145 l 1 1 1 7387 4~T1 0 25 0 26 99 13 0 19 0 17 8 957 Tl 20 I 8 178 20 764 9639

    system associated with the first detector permitted us to reject pulses separated by more than 100 nsec and less than 3/lsec This system was found to be the most efficient way of rejecting the elastically scattered 3He which otherwise would have produced pile-up in the electromcs This arrangement allowed the observaUon of protons from 3 to 5 MeV, deuterons from 4 to 7 MeV and alphas from 12 to 20 MeV Clearly then the regton above 10 MeV, which is the region of interest here, contained only alphas At hlgher angles, where scattered 3He are less numerous, we used a single 300/~m detectors wath a pile-up lejecuon s~stem The resolution of the system varied between

  • TllaHe, ~) REACTIONS 403

    75 to 100 keV during the course of the experiment, the resolution being slightly better

    with the second arrangement The targets were m the form of t itanium oxide evaporated onto a carbon backing

    The backing thickness was 50 pm and the evaporated layer thickness was about 75 ~m Ov~lng to the high temperature needed, we found it easier to float the carbon only after the evaporation The composit ion of the isotopic material was as Indicated

    in table 1 The 48T1 was present in all the data and since the 0 16 MeV state of 47T1 has a

    relatively large cross section it was identified and analysed in all the spectra This

    analysis proved to be very useful since all spectroscopic factors could then be nor- mahzed to this unique case Also, the alphas coming from this state were used to cah- brate the energy of the system Other cahbrat lon points were provided by the reac- tions 160(3He, ~)150 and 12C(3He, e)l 1C

    3. Experimental results

    The ~ particle spectra taken at 45 are shown in fig I The peaks have been identi- fied using the energy cal ibration points previously mentioned In many cases the ex- cited levels were observed by Kashy and Conlon 1) and we used their value of exci- tat ion energy since we did not have a better resolution The energies of the other peaks have been determined from the present data and they are accurate to +50 keV Those energies correspond to the 45TI 3 10, 4 79 and 5 78 MeV states, the 4VTl 3 45, 3 80 and 4 20 MeV states and the 49T1 4 20 MeV state The angular distr ibutions cor- responding to the various peaks are shown in figs 2, 3 and 4 The errors indicated are mainly of statistical origin but in some cases background subtraction contributes significantly to the lncertalnty and this has been Included in the error bars The ab- solute cross sections were estimated by comparison with the elastic scattering cross section on 46T1 This procedure is not very accurate since at 45 , the elastic cross sec- tion deviates from the Rutherford formula At smaller angles however, it was not possible to separate the peaks coming from contaminants in the target However, we measured the cross section for elastic scattering of 3He by 46T1 and our final accul acy was +25 %

    A DWBA analysis was performed using the Oak Ridge code T-Sally 6) In this calculation one uses a zero range approximat ion and does not include spin-orbit coupling The optical potential we used is of the following form

    U(r) = - V(e~+ l ) - J - tW(e x +1) -1+ Uc,

    where

    x = ( r - roA+) /a , ~c' = ( r - roA '~) /a '

    and U c is the Coulomb potential due to a uniform sphere of radius roc A~ Different sets of optical model parameters were used for the calculation, but only two sets were

  • 404 J L ' LCUYER AND C ST-PIERRE

    400_

    300

    200

    I00

    016 T j46(He3,~) T 45

    0 EHe 3 = I0 MeV ,1. :45 o

    elGb

    5 78 0

    I l , , t tml* ,~| ~, , -

    o -c o

    g u~

    g o

    4ooJ 300 i

    2001

    I O01 . . . . . . . .... ":.

    48 3 47 Ti (He,o

  • TJ(SHe, ~) REACTIONS 405

    found to gwe satisfactory fits to the data They are hsted in table 2 The first set is nearly identical to the one used by Chne et al 7) for their analysis of the 4Ca(3He, c 0 agca reaction at 10 MeV In the second set we took the entrance channel parameters from an analysis done by Yntema et al 8) of the 12 MeV 3He elasnc scattering on various T1 isotopes Various exit channel parameters, adapted from an analysis done

    0

    5 ~,,t~"~l 02

    0 2 ~ '14- f f t~ E: 0 OMen/ in = 3 OI

    OI

    005

    002

    001

    OO5 \

    I I I I

    o,I- t, mb/~rO 05

    002

    001

    -%

    1 I 1 I

    OI

    00 I

    O5 1

    02 Ex=O 55MeV

    In=2 OI

    0O5

    \\

    0 I ' ',

    005

    002

    00 I 0 o-

    ! l 20 40

    Ex ;3 IOMeV

    ' t t tit tt )i t

    I A L ! I

    t t f~i t

    i 1

    Ex= 4 79MeV

    E ~ 5 78 MeV

    0 2[ ~ Lr 0

    '\ 0 05 \ _

    t i IO 80 lu~ 60 1 80 1 IO0 0 2~0 40 ~ ~L~__~-

    Ocm ecm

    Fig 2 Angular distributions of the alphas from the reaction 46Tl('~He, a